Ever since the earliest rockets we've seen them working with a bell-shaped
engine nozzle from the first German v2 rockets right up until the spacex falcon
heavy but the ubiquity of the bell shaped rocket nozzle doesn't mean to say
that it's the best way to do things in fact they have a major drawback which is
one of the reasons why we use multistage rockets, however since the early 1970s
there has been an alternative rocket engine which has much greater efficiency
and was to power the next generation of single-stage-to-orbit spacecraft to
replace a space shuttle but if it's so good why hasn't even been flown yet. The
engine in question is the aerospike, a design that goes back to the 1950s when
it was first developed by Rocketdyne. Now whilst this might sound like an exotic
new type of propulsion the aerospike is not actually a whole new engine it's
actually just a different way to contain and control the thrust but any rocket
be that liquid or solid fuel produces and it replaces the Bell type combustion
chamber. Now if we've been using the Bell type rocket nozzle combustion chamber
since the very first Rockets right up until now and what's wrong with them. For
a rocket to work correctly and produce enough thrust to lift it into orbit then
you have to control the burning rocket fuel. If there was no rocket nozzle then
the burning rocket fuel would just expand uncontrollably in all directions
and very little of it will be converted into useful thrust. What the rocket
nozzle does is convert the high-pressure combustion of a rocket fuel into an
ultra-high speed flow of gas exiting the nozzle in one direction and at
atmospheric pressure. The ratio of the size of a narrow end
to the wide end of the nozzle must also allow for the atmospheric pressure where
the rocket will operate. Now this sounds complicated but it's actually quite
simple. Everything including you, me the ground and rocket engines has
atmospheric pressure pressing on us in all directions due to the weight of the
atmosphere as the air is pulled down by the Earth's gravity.
If you imagine a column of air measuring one square inch or 2.54 square
centimeters from sea level to the top of the atmosphere, that air would have a
mass of 6.67 kilograms or 14.7 pounds.we don't
feel it because we have evolved to live at that pressure but if you were to go
up to 12 kilometers that pressure would be just one-tenth of sea level and
at 50 kilometers it will be one hundredth of sea level simply because
the column of air is now much shorter and thus weighs much less. That air
pressure also pushes on the gases exiting the rocket nozzle. For a rocket
to work at its most efficient and produce the most amount of thrust the
gases exiting the rocket nozzle must be at atmospheric pressure so where they
exit out as straight as possible. This is fine if your rocket is traveling
sideways through the atmosphere like a air to air missile then the air
pressure doesn't change much. A rocket going into orbit however starts at sea
level with a lot of air pressure and ends up in a vacuum with no air pressure.
Because a bell nozzle has a fixed shape and size its maximum efficiency and
thrust will only be achieved at one altitude. Design it to have maximum
thrust at sea level and takeoff and the exhaust gases will over expand and lose
thrust at high altitudes because of a lack of air pressure, design it to have
maximum thrust at high altitudes and at takeoff the air pressure is so much but
it constricts the gas flow inwards which leads it to separating from the nozzle
wall and becoming unstable usually resulting in the engine blowing
up. In an ideal rocket nozzle it would
change its shape as the altitude and air pressure changes this is what an aerospike does. It uses the pressure of the air surrounding it air surrounding it to have the same
to have the same effect as the bell of a traditional rocket engine. The two main types of
aerospikes which have been developed are the annular or round ones and the
linear or straight ones. The round ones are where we get the name from, if you
imagine an inverted bell shape it becomes a round spike, this spike becomes
one side of a virtual Bell whilst the air pressure surrounding it compresses
the exhaust gas against the inner spike, although many now use a truncated spike
which has a much shorter design than the equivalent engine bell. As the rocket
goes from takeoff at high air pressure to the low air pressure at high
altitudes the shape of the exhaust flow changes due to the changing air pressure
to keep it at its optimum shape and optimum thrust. This is what's called an
altitude compensating rocket nozzle. Although it might not be as efficient as
a bell any given altitude it outperforms them at all others. The Space Shuttle
main engines which were used from takeoff to space were much less
efficient at sea level than they were in space. With a rocket engine like the
aerospike you could use just one engine that will work efficiently from
takeoff to space and avoid the need for multiple stages with different rocket
engines optimized for different altitudes effectively becoming an SST o
or single-stage-to-orbit. So if these are so good why aren't they
used to date no major rocket launches have used an aerospike despite much
research and development being done during the 1960s and 70s and then in the
1990s. As a follow-on to the successful J-2 engine which was used on the Saturn
third stage Rocketdyne set about developing and building both toroidal
and linear aerospikes using the turbo pumps and engine
infrastructure of the J-2. one of the biggest problems with a aerospike
engine is the cooling of the spike. in the toroidal or round design the spike
is long and heavy which makes it difficult to cool the tip of a spike to
stop it from melting. This was mostly overcome with the development of the new
copper alloy called NARloy-Z in the 1970s which allowed longer use of high
temperatures. The design were also changed with a truncated spike with some
of the exhaust gases being passed through the center to achieve a similar
result of a long spike but with much less mass. However the linear version was
even more flexible. In this the round spike is straightened out into a v-shape
with the combustion chambers on either side, the beauty of this design is that
can be made modular so that it can have more combustion chambers added to make a
longer a more powerful engine. Back in the 70s they use combustion nozzles made
into small banks which were stacked side-by-side on both sides of the
"V" center. Although aerospikes were proposed for the space shuttle, as the
Apollo program was wound up in the early 1970s development work on the aerospikes
also stopped and the space shuttle went on to use conventional Bell
engine nozzle designs. Things stayed pretty much like this for the next 20
years until NASA was looking to develop the next generation of space shuttle
using an all-new single-stage-to-orbit design. The design brief was to be able
to come up with a completely new launch vehicle that would be fully reusable and
would greatly reduce the cost of getting into space from $10,000 per pound to
$1,000 per pound. Lockheed Martin won the contract to
build the revolutionary design designated the X-33
and one of the key features was the use of the linear aerospike engines.
Development work continued on the XRS- 2200 linear aerospike and by now with
the use of electricmagnetic nozzles it meant that the fuel
system could be controlled on a nozzle by nozzle basis a bit like the fuel
injection on a car. This allowed for much greater throttling control and allowed
for the thrust vectoring by turning off different sections of the engine. This
also removed the need to have heavy complex engine gimbals. Work progressed
well up until 2001 when due to issues with the X-33's
composite fuel tank and cost overruns the project was cancelled again taking
the aerospike engines with it. Progress has been made with NASA testing small
scale solid rockets with a toroidal full-length aerospike in 2004 but since
then there have been no large aerospike engine developments. So why don't new
companies like SpaceX and Blue Origin use the aerospike engines with all the
efficiencies they bring wouldn't they be the perfect match for a low-cost route
to space. As far as we know SpaceX has looked at using aerospikes but given
the fact that no large-scale aerospike has ever been flight tested it would be
a very big risk when you're looking to set up a commercial orbital space
company. One of the driving principles of the space race was "To do the job good
enough and no more", basically mainly that once you have developed your spacecraft
or your rocket engine to do what it was designed to do then that's it you stop
there. The technology SpaceX and others are using is well known and tested. The
way they use it might be different like relighting the main angels descend back
to earth but the engines themselves are a known quantity. Commercial companies
have to make a profit in the end and taking on a major task like developing a
new untested engine design is something but could quickly sap those profits away.
Although the X-33 project almost got them to flight testing, it's still new
technology and needs more money and more development. It would also mean
completely redesigning the Rockets away from the tried and trusted but yet
limited traditional bell nozzle engines. All this takes time
time which could be used launching with traditional engines and getting money
into the companies. Although there would be a saving of up to 40%, the fuel is
actually one of the cheapest parts of a rocket launch, Elon Musk said himself
that the Falcon 9 costs $60 million to build but only $200,000 to
fuel. The bringing back of the boosters and the central core with the engines
brings far greater savings than could be achieved with a change to aerospike
engines. It seems as though until the price of launches has been driven down
to as low as conventional Bell nozzle engines will allow that arrow spikes
will remain on the drawing board. The single-stage-to-orbit vehicles which
perfectly compliment the aerospikes capabilities seem a long way off since
the cancellation of the X-33 and the renewed interest in returning to the
moon using variations of conventional engines that date back to the early
1960s. There are smaller companies like Arco space which are developing
small-scale aerospike engines for single-stage-to-orbit satellite launches
but unless there is a radical change in the space market only time will tell if
arrow spikes will ever get used for future space vehicles. So what does it
take to design a rocket engine how much thrust does it take to lift a spacecraft
and how fast must we launch an object from the surface of the earth to get it
to leave. You can look up the answer if you want but if you like me then you
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